If you invoke Perl with the -d switch,
your script runs under the Perl source debugger.
This works like an interactive Perl environment,
prompting for debugger commands that let you examine source code,
set breakpoints,
get stack backtraces,
change the values of variables,
etc.
This is so convenient that you often fire up the debugger all by itself just to test out Perl constructs interactively to see what they do.
For example:

perl -d -e 42

In Perl, the debugger is not a separate program as it usually is in the typical compiled environment. Instead, the -d flag tells the compiler to insert source information into the parse trees it's about to hand off to the interpreter. That means your code must first compile correctly for the debugger to work on it. Then when the interpreter starts up, it preloads a Perl library file containing the debugger itself.

The program will halt right before the first run-time executable statement (but see below regarding compile-time statements) and ask you to enter a debugger command. Contrary to popular expectations, whenever the debugger halts and shows you a line of code, it always displays the line it's about to execute, rather than the one it has just executed.

Any command not recognized by the debugger is directly executed (eval'd) as Perl code in the current package. (The debugger uses the DB package for its own state information.)

Leading white space before a command would cause the debugger to think it's NOT a debugger command but for Perl, so be careful not to do that.

If you supply another debugger command as an argument to the h command, it prints out the description for just that command. The special argument of h h produces a more compact help listing, designed to fit together on one screen.

If the output the h command (or any command, for that matter) scrolls past your screen, either precede the command with a leading pipe symbol so it's run through your pager, as in

Same as print {$DB::OUT} expr in the current package. In particular, because this is just Perl's own print function, this means that nested data structures and objects are not dumped, unlike with the x command.

The DB::OUT filehandle is opened to /dev/tty, regardless of where STDOUT may be redirected to.

Display all (or some) variables in package (defaulting to the main package) using a data pretty-printer (hashes show their keys and values so you see what's what, control characters are made printable, etc.). Make sure you don't put the type specifier (like $) there, just the symbol names, like this:

V DB filename line

Use ~pattern and !pattern for positive and negative regexps.

Nested data structures are printed out in a legible fashion, unlike the print function.

Next. Executes over subroutine calls, until it reaches the beginning of the next statement. If an expression is supplied that includes function calls, those functions will be executed with stops before each statement.

Set a breakpoint. If line is omitted, sets a breakpoint on the line that is about to be executed. If a condition is specified, it's evaluated each time the statement is reached and a breakpoint is taken only if the condition is true. Breakpoints may be set on only lines that begin an executable statement. Conditions don't use if:

Level of verbosity. By default the debugger is in a sane verbose mode, thus it will print backtraces on all the warnings and die-messages which are going to be printed out, and will print a message when interesting uncaught signals arrive.

To disable this behaviour, set these values to 0. If dieLevel is 2, then the messages which will be caught by surrounding eval are also printed.

affects printing messages on entry and exit from subroutines. If frame & 2 is false, messages are printed on entry only. (Printing on exit may be useful if inter(di)spersed with other messages.)

If frame & 4, arguments to functions are printed as well as the context and caller info. If frame & 8, overloaded stringify and tied FETCH are enabled on the printed arguments. If frame & 16, the return value from the subroutine is printed as well.

The length at which the argument list is truncated is governed by the next option:

Change style of string dump. Default value of quote is auto, one can enable either double-quotish dump, or single-quotish by setting it to " or '. By default, characters with high bit set are printed as is.

If set, goes in NonStop mode, and would not connect to a TTY. If interrupt (or if control goes to debugger via explicit setting of $DB::signal or $DB::single from the Perl script), connects to a TTY specified by the TTY option at startup, or to a TTY found at runtime using Term::Rendezvous module of your choice.

This module should implement a method new which returns an object with two methods: IN and OUT, returning two filehandles to use for debugging input and output correspondingly. Method new may inspect an argument which is a value of $ENV{PERLDB_NOTTY} at startup, or is "/tmp/perldbtty$$" otherwise.

If set, debugger goes into noninteractive mode until interrupted, or programmatically by setting $DB::signal or $DB::single.

Here's an example of using the $ENV{PERLDB_OPTS} variable:

$ PERLDB_OPTS="N f=2" perl -d myprogram

will run the script myprogram without human intervention, printing out the call tree with entry and exit points. Note that N f=2 is equivalent to NonStop=1 frame=2. Note also that at the moment when this documentation was written all the options to the debugger could be uniquely abbreviated by the first letter (with exception of Dump* options).

Other examples may include

$ PERLDB_OPTS="N f A L=listing" perl -d myprogram

- runs script noninteractively, printing info on each entry into a subroutine and each executed line into the file listing. (If you interrupt it, you would better reset LineInfo to something "interactive"!)

$ env "PERLDB_OPTS=R=0 TTY=/dev/ttyc" perl -d myprogram

may be useful for debugging a program which uses Term::ReadLine itself. Do not forget detach shell from the TTY in the window which corresponds to /dev/ttyc, say, by issuing a command like

Set an action (Perl command) to happen after the prompt when you've just given a command to return to executing the script. A multi-line command may be entered by backslashing the newlines. If command is missing, resets the list of actions.

where that number is the command number, which you'd use to access with the builtin csh-like history mechanism, e.g., !17 would repeat command number 17. The number of angle brackets indicates the depth of the debugger. You could get more than one set of brackets, for example, if you'd already at a breakpoint and then printed out the result of a function call that itself also has a breakpoint, or you step into an expression via s/n/t expression command.

If you want to enter a multi-line command, such as a subroutine definition with several statements, or a format, you may escape the newline that would normally end the debugger command with a backslash. Here's an example:

DB<1> for (1..4) { \
cont: print "ok\n"; \
cont: }
ok
ok
ok
ok

Note that this business of escaping a newline is specific to interactive commands typed into the debugger.

The left-hand character up there tells whether the function was called in a scalar or list context (we bet you can tell which is which). What that says is that you were in the function main::infested when you ran the stack dump, and that it was called in a scalar context from line 10 of the file Ambulation.pm, but without any arguments at all, meaning it was called as &infested. The next stack frame shows that the function Ambulation::legs was called in a list context from the camel_flea file with four arguments. The last stack frame shows that main::pests was called in a scalar context, also from camel_flea, but from line 4.

in $=main::BEGIN() from /dev/nul:0
in $=Config::BEGIN() from lib/Config.pm:2
Package lib/Exporter.pm.
Package lib/Carp.pm.
Package lib/Config.pm.
in $=Config::TIEHASH('Config') from lib/Config.pm:644
in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
in @=Config::myconfig() from /dev/nul:0
in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574

in $=main::BEGIN() from /dev/nul:0
in $=Config::BEGIN() from lib/Config.pm:2
Package lib/Exporter.pm.
Package lib/Carp.pm.
out $=Config::BEGIN() from lib/Config.pm:0
Package lib/Config.pm.
in $=Config::TIEHASH('Config') from lib/Config.pm:644
out $=Config::TIEHASH('Config') from lib/Config.pm:644
in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
out $=main::BEGIN() from /dev/nul:0
in @=Config::myconfig() from /dev/nul:0
in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
out $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/Config.pm:574

in $=main::BEGIN() from /dev/nul:0
in $=Config::BEGIN() from lib/Config.pm:2
Package lib/Exporter.pm.
Package lib/Carp.pm.
out $=Config::BEGIN() from lib/Config.pm:0
Package lib/Config.pm.
in $=Config::TIEHASH('Config') from lib/Config.pm:644
out $=Config::TIEHASH('Config') from lib/Config.pm:644
in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
out $=main::BEGIN() from /dev/nul:0
in @=Config::myconfig() from /dev/nul:0
in $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
in $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574

In all the cases indentation of lines shows the call tree, if bit 2 of frame is set, then a line is printed on exit from a subroutine as well, if bit 4 is set, then the arguments are printed as well as the caller info, if bit 8 is set, the arguments are printed even if they are tied or references, if bit 16 is set, the return value is printed as well.

If you have any compile-time executable statements (code within a BEGIN block or a use statement), these will NOT be stopped by debugger, although requires will (and compile-time statements can be traced with AutoTrace option set in PERLDB_OPTS). From your own Perl code, however, you can transfer control back to the debugger using the following statement, which is harmless if the debugger is not running:

$DB::single = 1;

If you set $DB::single to the value 2, it's equivalent to having just typed the n command, whereas a value of 1 means the s command. The $DB::trace variable should be set to 1 to simulate having typed the t command.

Another way to debug compile-time code is to start debugger, set a breakpoint on load of some module thusly

DB<7> b load f:/perllib/lib/Carp.pm
Will stop on load of `f:/perllib/lib/Carp.pm'.

and restart debugger by R command (if possible). One can use b compile subname for the same purpose.

Most probably you not want to modify the debugger, it contains enough hooks to satisfy most needs. You may change the behaviour of debugger from the debugger itself, using Options, from the command line via PERLDB_OPTS environment variable, and from customization files.

You can do some customization by setting up a .perldb file which contains initialization code. For instance, you could make aliases like these (the last one is one people expect to be there):

(the code is executed in the package DB). Note that .perldb is processed before processing PERLDB_OPTS. If .perldb defines the subroutine afterinit, it is called after all the debugger initialization ends. .perldb may be contained in the current directory, or in the LOGDIR/HOME directory.

If you want to modify the debugger, copy perl5db.pl from the Perl library to another name and modify it as necessary. You'll also want to set your PERL5DB environment variable to say something like this:

BEGIN { require "myperl5db.pl" }

As the last resort, one can use PERL5DB to customize debugger by directly setting internal variables or calling debugger functions.

As shipped, the only command line history supplied is a simplistic one that checks for leading exclamation points. However, if you install the Term::ReadKey and Term::ReadLine modules from CPAN, you will have full editing capabilities much like GNU readline(3) provides. Look for these in the modules/by-module/Term directory on CPAN.

A rudimentary command line completion is also available. Unfortunately, the names of lexical variables are not available for completion.

If you wish to supply an alternative debugger for Perl to run, just invoke your script with a colon and a package argument given to the -d flag. One of the most popular alternative debuggers for Perl is DProf, the Perl profiler. As of this writing, DProf is not included with the standard Perl distribution, but it is expected to be included soon, for certain values of "soon".

Meanwhile, you can fetch the Devel::Dprof module from CPAN. Assuming it's properly installed on your system, to profile your Perl program in the file mycode.pl, just type:

perl -d:DProf mycode.pl

When the script terminates the profiler will dump the profile information to a file called tmon.out. A tool like dprofpp (also supplied with the Devel::DProf package) can be used to interpret the information which is in that profile.

When you call the caller function (see "caller" in perlfunc) from the package DB, Perl sets the array @DB::args to contain the arguments the corresponding stack frame was called with.

If perl is run with -d option, the following additional features are enabled:

Perl inserts the contents of $ENV{PERL5DB} (or BEGIN {require 'perl5db.pl'} if not present) before the first line of the application.

The array @{"_<$filename"} is the line-by-line contents of $filename for all the compiled files. Same for evaled strings which contain subroutines, or which are currently executed. The $filename for evaled strings looks like (eval 34).

The hash %{"_<$filename"} contains breakpoints and action (it is keyed by line number), and individual entries are settable (as opposed to the whole hash). Only true/false is important to Perl, though the values used by perl5db.pl have the form "$break_condition\0$action". Values are magical in numeric context: they are zeros if the line is not breakable.

Same for evaluated strings which contain subroutines, or which are currently executed. The $filename for evaled strings looks like (eval 34).

The scalar ${"_<$filename"} contains "_<$filename". Same for evaluated strings which contain subroutines, or which are currently executed. The $filename for evaled strings looks like (eval 34).

After each required file is compiled, but before it is executed, DB::postponed(*{"_<$filename"}) is called (if subroutine DB::postponed exists). Here the $filename is the expanded name of the required file (as found in values of %INC).

After each subroutine subname is compiled existence of $DB::postponed{subname} is checked. If this key exists, DB::postponed(subname) is called (if subroutine DB::postponed exists).

A hash %DB::sub is maintained, with keys being subroutine names, values having the form filename:startline-endline. filename has the form (eval 31) for subroutines defined inside evals.

When execution of the application reaches a place that can have a breakpoint, a call to DB::DB() is performed if any one of variables $DB::trace, $DB::single, or $DB::signal is true. (Note that these variables are not localizable.) This feature is disabled when the control is inside DB::DB() or functions called from it (unless $^D & (1<<30)).

When execution of the application reaches a subroutine call, a call to &DB::sub(args) is performed instead, with $DB::sub being the name of the called subroutine. (Unless the subroutine is compiled in the package DB.)

Note that no subroutine call is possible until &DB::sub is defined (for subroutines outside of package DB). (This restriction is recently lifted.)

(In fact, for the standard debugger the same is true if $DB::deep (how many levels of recursion deep into the debugger you can go before a mandatory break) is not defined.)

With the recent updates the minimal possible debugger consists of one line

sub DB::DB {}

which is quite handy as contents of PERL5DB environment variable:

env "PERL5DB=sub DB::DB {}" perl -d your-script

Another (a little bit more useful) minimal debugger can be created with the only line being

sub DB::DB {print ++$i; scalar <STDIN>}

This debugger would print the sequential number of encountered statement, and would wait for your CR to continue.

The following debugger is quite functional:

{
package DB;
sub DB {}
sub sub {print ++$i, " $sub\n"; &$sub}
}

It prints the sequential number of subroutine call and the name of the called subroutine. Note that &DB::sub should be compiled into the package DB.

At the start, the debugger reads your rc file (./.perldb or ~/.perldb under Unix), which can set important options. This file may define a subroutine &afterinit to be executed after the debugger is initialized.

After the rc file is read, the debugger reads environment variable PERLDB_OPTS and parses it as a rest of O ... line in debugger prompt.

It also maintains magical internal variables, such as @DB::dbline, %DB::dbline, which are aliases for @{"::_<current_file"}%{"::_<current_file"}. Here current_file is the currently selected (with the debugger's f command, or by flow of execution) file.

Some functions are provided to simplify customization. See "Debugger Customization" for description of DB::parse_options(string). The function DB::dump_trace(skip[, count]) skips the specified number of frames, and returns an array containing info about the caller frames (all if count is missing). Each entry is a hash with keys context ($ or @), sub (subroutine name, or info about eval), args (undef or a reference to an array), file, and line.

The function DB::print_trace(FH, skip[, count[, short]]) prints formatted info about caller frames. The last two functions may be convenient as arguments to <, << commands.